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In his famous Summa Theologica, the Scholastic theologian Thomas Aquinas presents Five Ways to demonstrate the existence of God. Here is Aquinas' First Way, the argument from motion:

The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.

Here is the basic logic of the argument as I understand it:

  1. Motion, defined as the actualization of a potential attribute of an object, exists.

  2. The actualization of a potential attribute of an object cannot be caused by that attribute, because only what is actual can cause what is potential to become actual.

  3. Therefore, the motion of an object must be caused by some other object.

  4. If that other object is itself undergoing motion with respect to some attribute, this motion must be caused by some yet another object, and if that object is in motion then that motion must be caused by the still another object, etc.

  5. The immediate efficient cause of something is simultaneous with it, so we have an essentially ordered series of causes existing in the present moment. But essentially ordered causal series cannot go on forever.

  6. Thus the sequence must end with an object which is not being moved with respect to any of its attributes, an unmoved mover. And Aquinas defines this as God.

My question is about the reasoning involved in going from step 2 to step 3. What is the justification for step 3? Why can't the actualization of a potential attribute of an object be caused by an actual attribute of the same object? If that were the case, then I don't see how the infinite regress argument could possibly go through.

Any help would be greatly appreciated.

Thank You in Advance.

EDIT: My understanding of Aquinas' First Way is based on the works of Edward Feser. Here is a discussion of the First Way from his book The Last Superstition.

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I think your attempt to divide the argument into stages is not bad, but your wording for 1 and 2 causes you to run into some problems from the get-go:

  1. defined as the actualization of a potential attribute of an object

and

  1. The actualization of a potential attribute of an object cannot be caused by that attribute, because only what is actual can cause what is potential to become actual.

First, note that Aquinas does not speak of objects. This is because "object" at the time he's writing would have a very different meaning than to our post-Kantian conception. This also helps the "attribute" talk to be a little bit misplaced here. It's not potential attributes of the object, it's actual potentials in the thing. Or to put it another way, these are already in the thing as sight is already a potential of the eye even when my eyes are closed.

The second claim is also a little bit weirder as a translation for what Aquinas is saying. That might be closer to what's going on in the second way (efficient cause) than in the motion argument. The motion argument doesn't need such a complex notion of objects. It just needs (a) a definition of motion, (b) the fact of its occurrence, (c) an exploration of how the conditions that match the existence of motion and its definition can be met.

I generally formulate the argument roughly in this way:

  1. There is motion

  2. Motion is by definition A causes B to move.

  3. A and B cannot be identical because actuality of motion must be brought about by actuality of motion -- not potential to move in the object itself.

This is easy to misunderstand because we also speak of "potential energies" in physics, but even a ball at the top of a half-pipe is either (a) already in motion or (b) waiting for something to make it move.

If this set is right, we get two choices either an infinite series of prior As which Aquinas rejects as a vicious negative regress or some object that defies the normal definition of motion and causes its own motion.


There are several ways people can object to this argument, including (a) accepting the infinite regress as okay, (b) denying the validity of the definition of motion [often made under a somewhat suspect claim that QM has defeated this concept of motion -- made by people who still function under it when managing to type and submit things], or (c) suggesting multiple unmoved movers.

Also, a semi-unrelated move is to reject the identification of this unmoved mover with God.

  • @virmalor OK, how is this rephrasing of the (beginning of) the argument: 1. Motion (defined as an actualization of a thing's potential) exists. 2. The actualization of a potential cannot be caused by that potential. 3. The actualization of A's potential must be caused by something B which is not identical with A. Is that an acceptable paraphrase? If so, how do we go from step 2 to step 3? – Keshav Srinivasan Nov 2 '14 at 15:12
  • Why can't A cause the actualization of a potential of A? If A were to cause the actualization of one of its own potentials, why would that imply that that potential is actualizing itself? – Keshav Srinivasan Nov 2 '14 at 15:34
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    The problem is that the initial state is assumed to be stable. So no, if this inherent potential underwent to no other effect, it could not actualize itself, or it would have done so in the course of the preceding infinity of time. Were it not perfectly stable, however slowly it moved, it would have already completed its motion and become actual. To me, that is an unarticulated assumption. Why would this be stable? Physics has to assume that either singularity (via field theory) or vaccuum (via virtual particles) is always an implicitly unstable state, in order to get to the Big Bang. – jobermark Nov 2 '14 at 19:18
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    @jobermark Aquinas' First Way isn't about how the universe began, or about a series of causes going backwards in time. It's about a series of causes in the present moment, since one of the premises of the argument is that the immediate efficient cause of something is simultaneous with it. – Keshav Srinivasan Nov 2 '14 at 21:13
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    Well, in any intermediate context, it just kind of contradicts Newton's First Law. The actions upon the ball at the top of the hill, when it stops are in no way different from those happening the instant before it got there. There are no more and no fewer agents and they are not acting any more or any less than they already were (to any notable degree). So it is a potential (to pause) actualizing without a change in action. I understand inertia is counter to Aristotle, but it does make the initial case the only one that applies. – jobermark Nov 2 '14 at 23:16
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I think that Aquinas's references to "respects" imply that movement, according to him, is always caused per respect (i.e. per attribute). Heating can be caused by heat, sweetening can be caused by sweetness, but heating cannot be caused by sweetness, and sweetening cannot be caused by heat.

Why this interpretation? First, because otherwise, what would be the point of Aquinas's repeated emphasis on "same respect" in that paragraph?

"Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects"

"It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself"

Second, the two specific examples of causation that are mentioned in the paragraph, are of the same respect structure: the heat of the fire causes the heating of the wood; the movement of the hand causes the movement of the staff that is held in it.

"Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it."

"as the staff moves only because it is put in motion by the hand"

In summary, a thing cannot move itself, according to Aquinas, because it cannot move itself in any one respect R (because it cannot be both a mover and a moved in respect R - because a mover would be actual in respect R, and a moved would be potential in respect R. And a thing cannot be both actual and potential in the same respect).

What do you think?

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I would say that this argument from Thomas Aquinas was refuted, or at least made obsolete, starting with the 17th century with the work of Galileo Galilei and Sir Isaac Newton that gave us a better understanding of what motion is and why things move. Our understanding of motion has continued to improve, and there are many paths of objection.

One of the quickest refutations of Aquinas's train of logic comes from our understanding of gravity. If you imagine the universe to be completely empty of everything save for two bodies with non-zero mass initially at rest with respect to each other, we know that these two bodies will exert a gravitational tug on each other. Each will cause the other to move towards their common centre of mass. Body A causes Body B to move, while simultaneously Body B causes Body A to move. Neither self-moving nor infinite regress required - motion is a fundamental property of the universe at inter-planetary scales, and it is rest that is unstable.

We also now know that the natural state of the smaller things in nature nature is not to be at rest, but in motion. This seems weird to our macroscopic common-sense where we're constantly fighting against friction, but is a fundamental feature of our universe at the molecular level. So long as there exist temperatures above absolute zero there will be motion. Observing the Brownian motion of dust particles in a sunbeam on a contemplative summer day shows that complex macroscopic behaviour can emerge from this random molecular noise, too.

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    To my mind, the 'quick' refutation is overstated. Gravity is still an action each body exerts on the other. And the argument still survives in a relativized version where change in motion is the relevant question, over motion itself. This analysis just pushes the question back earlier in time. I think Newton and Galileo, both very orthodox, were fine with this argument from such a 'divine watchmaker' point of view. It still requires a stable initial state, which, as you note, more modern physics rejects. – jobermark Nov 3 '14 at 21:46
  • The two-body gravitational system does involve two objects exerting forces on each other, and that's the point: If you remove all other forces from the equation and just place two lumps of matter in space they will accelerate toward each other. From the quote (paraphrase?): Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. **But this cannot go on to infinity...** - and a simple 2-body system shows that this is a false dichotomy. – Dave B Nov 3 '14 at 21:54
  • OK, as written this is a problem, but that is just chop-logic. You can just change that to "whatever changes motion must be have that motion changed by another interaction (perhaps built up over a limited time), and that one must be affected by another...". Unfortunately that can go on to infinity. But if there is a start to things from an initial stable state, that regress is not infinite. Newton thought it did, so he was not threatening orthodoxy with his laws. – jobermark Nov 3 '14 at 22:08
  • Basically what is wrong is the physics, not the philosophy, and if you just put Newton's laws in place of Aristotle's, the philosophy remains sound. – jobermark Nov 3 '14 at 22:12
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    Obviously, it fails the notion of virtual particles. But that is on the other side of a huge discontinuity of worldview, greater even than Newton's, with real philosophical earthquakes involved, not just moderate paradigm shifts. – jobermark Nov 3 '14 at 22:14

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